1. Field of the Invention.
This invention relates to multipoint data modem communication systems, of the kind including a control modem and a plurality of tributary modems, wherein said tributary modems include respective modem transmit means adapted to transmit a training signal and a modem identification signal and said control modem includes tributary modem identification means.
2. Description of the Prior Art.
A data modem communication system of the kind specified is known from European patent application No. 0 169 548. According to the known system, a master modem is connected in a multipoint network with a plurality of remote (tributary) modems. During an initial, relatively long, training sequence, receiver operating parameters and equalizer coefficients are acquired and stored at locations allocated to the respective remote modems. During subsequent, relatively short training sequences, the previously stored coefficients and parameters are acquired from storage and adjusted, if adjustment is required. For these subsequent, short training sequences, the remote modems transmit identification codes. According to a first arrangement, the identification code is formed by a time interval which is transmitted after the training sequence but before the data is transmitted. According to a second arrangement, the identification code is formed by a tone frequency which is transmitted before the training sequence and the data is transmitted. Each remote modem may have a respective tone frequency associated therewith, or there may be a sequence of different tone sets to identify the modem. The known system has the disadvantage that the start-up time for transmissions from the remote modems is lengthened as a result of the transmission of the modem identification signal in sequence with the training signal.